Determination of the fine details of hair-cell function is crucial to our understanding of the mechanisms of normal hearing. A healthy human cochlea contains tens of thousands of hair cells, and young humans can detect sound pressures as low as 2x10-5 Pa and frequencies of up to 20 kHz. Thus, hair cells are remarkably fast and incredibly sensitive. Loss of sensitivity of the hair cells can lead to irreversible loss of hearing. A better understanding of hair cell function may facilitate design of rational strategies for amelioration of hearing deficits, common health problems that affect tens of millions of Americans. The sensitivity of hair cells is achieved by the cells' ability to adapt to mechanical displacements, whereby hair cells adjust their responsiveness to further stimuli following an initial displacement. Considerable circumstantial evidence suggests that myosin Ibeta is the adaptation motor of hair-cell transduction. Myosin is thought to interact with actin molecules in hair cells to induce adaptation. The experiments proposed in this application are designed to test this hypothesis directly. We intend to identify specific and selective inhibitors of this enzyme and use them to disengage adaptation motors. We will address three principal issues that will directly prove or disprove that myosin Ibeta is the adaptation motor. First, what specific compounds can disengage the adaptation motor from their actin substrates? We expect that phosphate analogs will disrupt interaction of myosin and actin and therefore block adaptation. Second, what specific peptides can disrupt interaction of myosin Ibeta with actin in hair cells? We will purify myosin Ibeta and identify specific peptides that disrupt myosin Ibeta function. Finally, will introduce these peptides into hair cells and determine whether they block adaptation in hair cells. These experiments should ascertain unequivocally whether myosin Ibeta mediates adaptation in hair cells.